A conventional DC motor is constructed in such a manner that a field coil (exciting coil) is wound round a rotor, having an appropriate pole number, and a coil having a brush is wound round the rotor. The conventional DC motor requires the brush to be replaced due to short-circuiting or abrasion of segment bars.
Furthermore, in a conventional AC motor, DC motor and small-capacity BLDC motor, controllers for controlling the operations of these motors are set integrally with the motors, and thus it is difficult to manufacture, repair and maintain the motors. A controller of the conventional AC motor has a large size and is expensive and difficult to control because it executes high-voltage low-current control. A controller of the conventional DC motor performs low-voltage large-current control so that it is difficult to manufacture due to flash phenomenon of the DC motor. In addition, switching elements of the controller of the conventional DC motor is expensive because the controller executes large-current control. Moreover, since the conventional motors generate a surge voltage, reactance, harmonics and so on, the controllers of the conventional motors are difficult to design and expensive.
Furthermore, the conventional integral type controllers cannot independently operate because of characteristics of motors, and thus a system employing a conventional motor is not operated when the motor or its controller is abnormal and the emergency operation of the system cannot be performed.
In addition, a conventional motor control method uses a sine-wave signal, a square-wave signal or a trapezoidal wave signal so that a counter electromotive force is generated when a motor is started and stopped. Thus, it is difficult to construct and control a controller of the motor. Furthermore, when current flows through the controller, arm short of switching elements (switching elements are simultaneously turned on to result in short-circuiting, which destroys the switching elements) is generated. To prevent this, a crossfire prevention circuit must be constructed using hardware or software.
Moreover, the conventional motor control method simultaneously turns on and off switching elements at upper and lower stages of a motor. When the switching elements are simultaneously turned off, remaining power (hysteresis phenomenon) is left in a motor coil. This remaining power and an input voltage cause an abnormal phenomenon such as a surge voltage when the switching elements are turned on. To prevent this, a circuit is constructed using hardware or software, and thus a drive size increases, motor control becomes difficult and the controller becomes expensive.
Furthermore, the conventional motor control method does not require synchronization because it does not use an electronic motor (it uses a closed circuit control method for linear control or constant-speed control). Moreover, linear control is difficult to perform and a high-grade algorithm is required for constant-speed control in the conventional motor control method. In addition, a small-capacity BLDC motor is driven by a sensor signal because it is an electronic motor. However, the small-capacity BLDC motor employs Y wire-connection and delta wire-connection as a wire connecting method so that its speed is controlled using a general AC motor control method. Accordingly, it is difficult to control the speed of the small-capacity BLDC motor.